code: def test_gco_dark_zone():
      def square(x):
          return np.sum(x**2)
  
      search_space = search.SearchSpace(
          n_agents=4,
          n_variables=2,
          lower_bound=[1, 1],
          upper_bound=[10, 10],
      )
  
      new_gco = gco.GCO()
      new_gco.compile(search_space)
  
      new_gco._dark_zone(search_space.agents, square)
  
file_path: opytimizer/tests/opytimizer/optimizers/population/test_gco.py
imports: import numpy
  from opytimizer.optimizers.population import gco
  from opytimizer.spaces import search
label_desc: Use image node_id 5 for calling a global function with example usage: test_gco_dark_zone() and returns: np